This invention pertains to a fail-safe pressure override control for a variable displacement pump which utilizes the high flow available from the variable displacement pump to rapidly change the pump displacement when the system pressure exceeds a predetermined value and thus avoid undesirable pressure overshoots. The pressure override control includes certain components which may malfunction and means are provided to prevent any such malfunction from causing the pump displacement to move toward its maximum value in an uncontrollable manner.
A simple form of pressure control for a pump is utilization of a pressure relief valve in the pump output line which will open when a predetermined pressure is exceeded and permit flow of system fluid to tank. This system results in generation of heat in the fluid in the circuit and also loss of power. It is also known to limit maximum system pressure supplied by a variable displacement pump by destroking the pump to minimize the loss of power and heat generation. When this is done with control fluid supplied at a relatively low flow rate by a charge pump, the system pressure may overshoot a desirable value because the movement of the pump displacement mechanism may require too much time with the force available from fluid provided by the charge pump. In order to overcome this, various systems have been devised which port fluid from the main system pump outlet into the displacement-controlling servo mechanism to use the high flow available to rapidly destroke the pump and avoid undesirable overshoot of system pressure beyond a desirable value. These systems override the normal servo mechanism for displacement control and destroke the pump to the amount required to maintain the set pressure level.
Examples of the foregoing systems utilizing fluid from the system pump for reducing pump displacement include the systems shown in Ferris et al U.S. Pat. No. 1,974,138 and Jennings U.S. Pat. No. 3,659,963.
The literature discloses that at least two manufacturers offer pressure override controls for a variable displacement pump as usable in a hydrostatic circuit wherein the displacement of the pump can be controlled through a displacement control valve and when an excessive system pressure occurs, valve components will act to direct pump outlet flow to a displacement-controlling servo motor to cause reduction in displacement of the pump.
In all of the foregoing systems disclosed in the patents and literature referred to above, there is a possibility of certain system malfunctions which, upon occurrence thereof, would cause the override control to override the normal displacement control and which could drive the pump to its maximum displacement in an uncontrollable manner. This is inconvenient and, in some cases, could result in dangerous operation as when the hydrostatic transmission utilizing the pressure override control is used in the drive for a vehicle.